The Susan and Diana Au oxide mineralization of Corihuarmi are associated with epithermal deposit types. Epithermal gold deposits form in hydrothermal systems related to volcanic activity. These systems, while active, discharge to the surface as hot springs or fumaroles.

Epithermal gold deposits occur largely in volcano-plutonic arcs (island arcs as well as continental arcs) associated with subduction zones, with ages similar to those of volcanism. The deposits form at shallow depth, <1km, and are hosted mainly by volcanic rocks.

There are two end-member styles of epithermal gold deposits, high sulfidation (HS) and low sulfidation (LS). The two deposit styles form from fluids of distinctly different chemical composition in contrasting volcanic environment. The Diana and Susan deposits are of the HS style.

Mineralization identified to date within the Corihuarmi Project comprises a high sulphidation epithermal precious metal system that formed at relatively shallow depth. Gold and silver mineralization is essentially confined to remnant zones of silicification and brecciation that dominantly lie along the northeast margin of the volcanic complex. Horizontal metallogenic zonation provides evidence that this siliceous layer was once continuous, effectively capping the hydrothermal system.

The most significant mineralization is associated with the Susan and Diana zones, which comprise resistant remnant mineralized silicified bluffs separated by some 180m. The Susan deposit measures approximately 200m by 350m in size, being confined at the margins by cliffs. The smaller Diana Zone is approximately 150m by 250m in area, and to some extent remains open to the northwest and southeast along the ridge-line. The siliceous layer is shallow dipping to sub-horizontally disposed, ranging in thickness from 10m to 75m, and averaging approximately 45m.

Gold mineralisation is mined by open pit methods at a mine production rate of 9,000 tonnes per day (t/d) of ore and 5,000 t/d of waste. The average grade produced in the last year (2017) is around 0.27 g/t and it is expected 0.28 g/t will be produced in the following years (2018 to 2020).

Mining at Corihuarmi uses a conventional truck and excavator configuration. Open pits are mined on 5-meter-high benches through drilling, blasting, loading and hauling unit operations. The material transportation circuit is performed in two parts; the first one is from the pit to the crusher and the second one from the crusher to the leach pad. If the ore does not require additional crushing, the material is transported directly to the leach pad. The waste is transported to the waste dump.

The mine schedule was produced in monthly periods and is based on 7 small pits which will be mined throughout 3 years of mining operations.

The Project is a heap leach operation utilizing a multiple-lift, single-use leach pad. Prior to placing the ore onto the leach pad the ore is primary crushed. Processing of ore began in January 2008 when irrigation of the heaps was started.

Ore from the mine, with a moisture content of 4 to 5% is transported by trucks to the Run of Mine (ROM) pad. The ore is then either dumped directly into the coarse ore bin (COB) or can be placed on the ROM pad and fed into the COB by front end loader.

Ore is withdrawn from the COB by a vibrating scalping grizzly feeder with 100mm gap size. The plus 100mm is fed into the primary jaw crusher. The minus 100 mm grizzly undersize material and the crusher product both drop onto the jaw crusher discharge conveyor. As the ore travels along the conveyor it is weighted and lime is added. From this conveyor the ore is discharged onto a stacking conveyor and is stockpiled. The crushed material is reclaimed using a front end loader and trucks and transported to the heap leach pad.

Heap Leaching is carried out in a single stage system. Pregnant Leach Solution (PLS) is delivered to activated carbon contactors to remove the gold after which the solution is pumped back to the heap leach pad. The activated carbon in the contactors is stripped from the carbon in a Zadra elution plant and the gold is electrowon onto cathodes. Sludge is removed from the cathodes and retorted to recover mercury prior to being direct smelted to recover the gold into bullion ready for sale.

The current heap leach pad covers an area of nine hectares and is designed to treat 6000tpd. Trucks enter the leach pad via a ramp at one end and dump the ore within one to two metres from the edge of the heap. A bulldozer is then used to push the material over the edge to achieve an even surface over the top at the lift height of eight metres.

The leach pad has a polyethylene geomembrane layer on the base and is divided into cells with an average area of 3,000m². Each cell holds 40,000 tonnes of ore.

Irrigation of the cells begins with the primary pipe network, consisting of a barren solution tank, 250HP pump and 10” Ø HDPE. This line is located in lined channel and runs to a manifold at the top of the heap. From the manifold the secondary pipe network completes the irrigation. The secondary pipe network consists of a 4” Ø Lay Flat pipes running down the leaching cells. Each Lay Flat pipe has hoses branching from them with drippers for irrigation.

The heap is irrigated at a rate of 10Lt/h/m². Caustic soda is added to the barren solution tank to maintain the pH of the solution between 10.0 and 10.5 to avoid cyanide losses. Cyanide is also added to the barren solution tank to maintain a concentration of 250 ppm the solution that percolates through the heap is collected by the underdrainage piping network which is connected to two HDPE pipes that transfer the solution to the Pregnant Leach Solution (PLS) pond. From the PLS pond the solution is pumped to the adsorption circuit. The adsorption circuit is filled with activated carbon. The activated carbon adsorbs the gold from solution and the barren solution (BS) exiting the adsorption circuit gravity flows back to the barren solution tank and is used for re-irrigating the heap again. The leaching cycle is completed in 90 days.

Anti-scalant is added to the barren solution to reduce the formation of scales on the irrigation pipes, heap top and the activated carbon. An excess solution (storm water) pond is included to contain leach solution in excess of that required for normal operations due to storm events. Excess solution will ultimately return to the barren tank as makeup solution to replace water lost in the process due to evaporation and wetting of the ore.

The present adsorption plant is capable of treating 250m³/h of solution pregnant. The circuit consists of two trains of 5 cascade columns each configured in series each containing 2t of activated carbon. The activated carbon adsorbs the cyanided gold complexes, silver and other metals in solution to remove them from the solution. When the carbon is loaded up 8,000g/t Au in the first carbon column, the carbon is removed from the column and sent to the elution circuit. Once this column is empty of carbon, carbon from the second column is loaded until it also reaches approximately 8,000g/t Au. All of the columns in the circuit will be cycled in series in this manner, with any of the columns being able to used to load gold prior to stripping. The last tank in the series is always filled with activated carbon that has been stripped and reactivated.

Stripping of the gold from the carbon (elution) is conducted in a 2t Zadra circuit at a temperature of 135°C. Barren solution, from the desportion tank, is pumped through a secondary heat exchanger and then into the primary heat exchanger prior to entering the column. The solution exiting the top of the column passed through the other side of the secondary heat exchanger, through a cooling heat exchanger to drop the temperature to 65°C. After cooling the solution passes into the electrowinning cell where the gold and any silver that is present is electrowon. The solution exiting the cell gravitates into the desorption tank and is then reused for stripping the gold from the carbon. The total time for desorption ranges between 14 hours and 16 hours.

The activated carbon is reactivated in a regeneration kiln followed by acid washing prior to being cycled back to the adsorption columns for further adsorption of gold and silver.

The electrowinning cell contains 18 stainless steel cathodes and 19 anodes. The loaded cathodes are washed with high pressure water to remove the gold and silver from the surfaces. The precious metals are collected and filtrated and placed in a retort to remove any mercury that was collected in the process. The mercury free material is then smelted in a tilting furnace to produce Doré.